1. Field of the Invention
This invention relates to the optical device of a copying apparatus, and more particularly to a supporting device for lens or mirror.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings schematically shows an example of the electrophotographic copying apparatus. The image of an original to be copied placed on an original carriage 1 moved in the direction of arrow is projected upon a photosensitive drum 5, rotated in the direction of arrow in synchronism with the original carriage 1, by a projection optical system comprising a plane mirror 2, a lens 3 and a plane mirror 4. The electrostatic image formed on the photosensitive drum 5 by the image projection is developed with toner. The developed image is transferred to paper. The toner image transferred to the paper is treated for fixation while, on the other hand, the drum 5 after the image transfer is cleaned.
In the copying apparatus as described above, the support the lens at a predetermined position in the original image projection light path, there has been a method using a casting bed having a V-shaped groove or an arcuately concave groove. However, this method encounters difficulties in providing the accuracy of the groove and is costly. It is also apt to create irregularity of the accuracy among individual devices. Also, in the case as shown in FIG. 2 of the accompanying drawings wherein a metal sheet is formed into a V-shaped or an arcuately shaped groove and a cylindrical lens barrel is received in such groove, the cost involved is low but the sheet thickness differs from one sheet to another and therefore, in each individual device, there occurs a bend dimensional error corresponding to the sheet thickness error .DELTA.t and the error .DELTA.x (distance between O and O') of the lens axis from its regular position with the hatched portion as the standard becomes .DELTA.x=.DELTA.t/(cos .theta./2), thus creating irregularity.
Such poor accuracy of the lens position lead to unclearness of resultant copies and this in turn leads to the inconvenience that complicated and cumbersome fine adjustment must be effected in each individual device.
Also, the usually practised method of mounting a plane mirror in the copying apparatus as shown in FIG. 1 is to adhesively secure or hold the back side of the reflecting surface of the mirror to or against the bent surface portion of two plate members, but this method encounters difficulties in keeping the accuracy of the angle of the bend thereof or of the relative angle between the two plates and requires fine adjustment in each individual device during the mass projection of devices and thus, it is inefficient. That is, a bend mold is usually made so as to correspond to the maximum value of the allowable error of the plate thickness, irregularity occurs to the right angle of the bend due to the irregularity of the plate thickness. In FIG. 3 of the accompanying drawings, if the plate thickness irregularity is .DELTA.t and the length of the bent portion is L, then the bend error .DELTA..theta. becomes sin .DELTA..theta.=.DELTA.t/L and this optically much far exceeds the tolerance, so that the planarity of the mirror M cannot be maintained. Also, if the angle between the bends of the two plates differs, the mirror will become distorted and again, sufficient planarity of the mirror cannot be maintained. In such case, resultant copy images will become unclear.
To offset such disadvantage, a method of using a casting as the mounting bed and milling the mirror mounting surface or a method using a diecast mounting bed has been attempted, but sufficient accuracy of inclination and planarity could not be obtained and it has finally been necessary to effect complicated and cumbersome fine adjustment in each individual device.